This chapter reviews the renal effects of radiological contrast media and pretreatment options in the patient with underlying renal insufficiency. Many patients will report an allergic reaction to contrast media. Diagnostic imaging, in general, exposes a patient to such a minimal dose of radiation that ED physicians often order multiple imaging tests on the same patient without regard to the radiation dose the patient receives. However, when a patient is pregnant or very young, the radiation exposure is more concerning, and the necessity of the imaging test must be balanced by the risk of the dose of radiation. In patients with renal insufficiency, diabetes, adverse reactions to contrast, and pregnancy, CT scans are not completely without risk. A careful consideration of these risks balanced with the clinical benefits of the diagnostic study should guide clinical decision making when considering CT scans in these patients.